Herpes simplex virus type 1 (HSV-1) is a common oral and genital pathogen that is also a leading cause of kerato-conjunctivitis leading to blindness and encephalitis. More serious and disseminated disease occurs in immune deficient individuals including neonates and AIDS patients. Encephalitic (neuroinvasive disease in the mouse model) and neonatal disease production are complex processes. It is not known which host selective pressures (e.g. the immune response, the target tissue (neuronal and other cells)) are important for selecting neuroinvasive disease determinants. As a result, naturally selected disease producing HSV-laboratory strains hold the clues to understanding the virulence mechanisms of HSV-1. Upon comparison of clinical isolates and attenuated laboratory strains, a strong correlation between neuroinvasive disease potential and the small plaque phenotype (tissue culture behavior characterized by small plaque production, a block in glycoprotien processing and inefficient virus release) was noted. The small plaque phenotype was mapped to near the ICP34.5 gene and genetic differences in ICP34.5 correlate with virulence and attenuation of neuroinvasive disease. The prototype virus (SP7) is a neuroinvasive, small plaque clinical isolate obtained from the brain of a neonate. Passage of SP7 yielded viruses (SLP5, SLP10) that are attenuated, produce large plaques and have and ICP34.5 gene that resembles the highly passaged, attenuated KOS virus strain. These and other studies indicate the importance of the ICP34.5 gene and protein for neuroinvasive disease and suggest that difference in the ICP34.5 gene and protein change the nature of the virus-cell and virus-host interactions. The purpose of this study is to further our understanding of the structure and function of ICP34.5 and its role in the neuroinvasive disease process and as a result gain a better understanding of HSV interactions with cells and the host. The unique aspect of these studies will be in comparison of variants in ICP34.5 generated by natural selection of HSV-1 in the human host, by tissue culture passage or by insertion of ICP34.5 gene sequences from SP7 into SLP5 or SLP10. They will compare the behavior of the ICP34.5 variants in neuronal and other cells and a mouse model of encephalitis and also the protein expressed from eukaryotic expression vectors containing PCR amplified ICP34.5 gene sequences from the same variants.